There are known a variety of methods for determining the motion vector, of which a technique for detecting a motion vector by using a gradient method will be described by reference to FIG. 3. For simplification of the description, only one dimension corresponding to x-direction will be considered. In FIG. 3, it is assumed that a picture signal 102-1 of an n-th field as indicated by a solid line has moved by .DELTA.x to the left from the position of a picture signal 102-2 of an (n-1)-th field indicated by a broken line. In this conjunction, if a field difference, i.e. an inter-field level difference .DELTA.t of the picture signal at a pixel point A relative to a same pixel point B can be known together with a spatial differential value, i.e., a value (g=tan .theta.) on the basis of a gradient at the pixel point A or C, magnitude of the motion of the picture taken place during one field, i.e., the motion vector can be determined. Expressing mathematically, it applies valid that .DELTA.x=.DELTA.t/tan .theta.=.DELTA.t/g. This is the basic principle underlying the gradient method. At this juncture, it is necessary that the gradient remains same at a pint C corresponding to the point A in order that the above-mentioned expression applies valid perfectly. If the above expression can apply valid as an approximate equation, it is required that the value of .DELTA.x be small. In general, however, the motion vector to be determined cannot always be small.
In the foregoing, the gradient method has been described on the basis of the picture signal. Next, a method of determining the motion vector on the basis of pictures as generated on a display screen or a picture plane will be elucidated by reference to FIG. 7. The picture plane is divided into a plurality of blocks with and blocks in the x- and y-directions, respectively.
In the case of the example illustrated in FIG. 7, the display picture plane for display is divided into 16 blocks with 4 blocks in each of the x- and y-directions. An object D0 FIG. 7 displayed within a block 200-33 in a preceding field is displayed as D1 within a block 200-22 in the current field. The motion of the object at this time can be represented by a motion vector V. For detecting this motion vector V, there is generally adopted a block gradient method. According to this block gradient method, a picture plane is divided into a plurality of blocks, wherein a motion vector to a given one of the blocks is determined by using inter-field or inter-frame signal differences at a plurality of pixels belonging to each block.
In other words, the motion vector V can be expressed as follows: ##EQU1## Let's consider an x-directional component and a y-directional component of a motion vector on an assumption that K pixels exist in each block. When representing by .DELTA.t.sub.i an inter-field picture signal difference between the preceding field and the current field at an i-th pixel and representing by g.sub.xi and .sub.yi the gradients in the x- and y-directions at the i-th pixel in the current field, respectively, the x-component V.sub.x and the y-component V.sub.y of the motion vector V can be expressed as follows: ##EQU2## where symbol .SIGMA. indicates summation for all the pixels within a given block. Once the x-component and the y-component have been determined, the motion vector V of a two-dimensional picture can easily be determined.
As described previously, when the motion vector V of a television picture is to be determined by the gradient method, a spatial band limitation is applied to the picture signal so that the picture signal has a same gradient at one and the same place between different fields or frames with a view to increasing detection accuracy. Further, when an interlaced television signal is to be processed by the gradient method, those signal components which are modulated and folded back by an interlacing carrier are diminished through a sequential scanning. In this conjunction, the circuit for the sequential scanning performs generally an intra-frame processing for a still picture portion while performing an intra-field processing for a motion picture portion.
Elucidation will now be made of signal waveforms making appearance when the spatial band limitation is applied to an input picture signal. In FIG. 4A, there are illustrated light signals impinging into a TV camera at a field interval. In this figure, a solid line, a broken line, a single-dot line, a double-dot line and a triple-dot line represent signals at an n-th field, (n+1)-th field, (n+2)-th field, ((n+3)-th field and an (n+4)-th field, respectively. As can be seen in the Figure, a rising portion of the edge moves toward the right. With a conventional TV camera, an output signal therefrom will be of such waveform as illustrated in FIG. 4B because of accompaniment of a storage effect. In the case of the signal shown in FIG. 4B, there exists no area in which picture signals of different fields overlap each other at same pixels on a picture plane, differing from the example illustrated in FIG. 3, which means that the conditions for validating the gradient method mentioned above are not fulfilled. Accordingly, without any modification, it is impossible to apply the gradient method. Under the circumstances, the spatial band limitation has heretofore been performed on the picture signal in an effort to increase the overlap area or region. A picture signal waveform resulting from the spatial band limitation is illustrated in FIG. 4C. By performing the spatial band limitation, coincidence can be established to some extent as to a range of computation points at which the motion vector is to be determined. However, the spatial band limitation alone has not always been sufficient for determination of the motion vector but often provided a cause for degradation in the detection accuracy of the motion vector because of presence of such rising portions of the edges in which no overlap is found, as can be seen in FIG. 4C.
Besides, in the case of the prior art block gradient method, the denominator of the expressions (1) and (2), i.e. the sum of absolute gradient values, assumes a value approximating to zero in a region where the gradient is small, as is obvious from the expressions (1) and (2). As a consequence, the value of the detected motion vector becomes significantly greater than the true or intrinsic motion vector value, thus presenting a problem that remarkable error is involved.
For making decision as to whether a motion vector determined by a given method is appropriate or not, it has heretofore been general to use an inter-frame difference between a picture moved on the basis of the motion vector and a picture actually moved. This is because the inter-frame difference becomes minimum when the motion vector has an optimal value, representing coincidence between the two pictures mentioned above. The decision as to the appropriateness of the motion vector as determined has heretofore been made by making use of the fact mentioned above. For the motion vector detection, a rigid object motion is presumed, wherein the motion vector is determined on the basis of relation between two or more pictures. More specifically, when a picture within a block of concern on a picture plane or screen displaying a motion picture is moving uniformly, i.e., when all the pictures appearing within that block have a same motion vector, the inter-frame difference between the pictures moved on the basis of that motion vector will become zero unless perturbation or fluctuation due to spurious noise or the like is taken into consideration. In actuality, however, because of the influences of change in luminance, presence of a boundary between a moving object and a still portion, within a block or the like the uniform picture motion can not always be expected. Additionally, an occlusion problem and uncovered background problem contribute to occurrence of significant inter-frame difference, making it difficult to decides whether the motion vector as determined is optimal or not. On the other hand, there may arise such situation in which the inter-frame difference decreases notwithstanding of the fact that the picture shifted based on the motion vector includes a portion which differs from the original picture. Thus, it is not always safe to say that the hitherto known motion vector detection method based on the inter-frame difference is to be appropriate or satisfactory.
Heretofore, detection of a scene change in a television picture is carried out in such manner that inter-frame or inter-field differences in the picture signals is determined over a plurality of regions on a picture plane, wherein when the difference exceeds a certain threshold value, motion of the picture is decided, and when the regions for which the motion of the picture is decided exceeds a certain proportion of one scene, the scene change of the picture is detected. With the hitherto known method of detecting the scene change in the television picture, the scene change occurring in the picture can certainly be detected with high reliability. However, the scene change detecting method is disadvantageous in that upon appearance of many picture signals representing motion in a scene due to panning and tilt, the scene change is detected erroneously.